cmd/tailscale/cli,ipn,all: make peer relay server port a *uint16

In preparation for exposing its configuration via ipn.ConfigVAlpha,
change {Masked}Prefs.RelayServerPort from *int to *uint16. This takes a
defensive stance against invalid inputs at JSON decode time.

'tailscale set --relay-server-port' is currently the only input to this
pref, and has always sanitized input to fit within a uint16.

Updates tailscale/corp#34591

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/udprelay/server.go

@@ -309,7 +309,7 @@ func (e *serverEndpoint) isBound() bool {
 // onlyStaticAddrPorts is true, then dynamic addr:port discovery will be
 // disabled, and only addr:port's set via [Server.SetStaticAddrPorts] will be
 // used.
-func NewServer(logf logger.Logf, port int, onlyStaticAddrPorts bool) (s *Server, err error) {
+func NewServer(logf logger.Logf, port uint16, onlyStaticAddrPorts bool) (s *Server, err error) {
 	s = &Server{
 		logf:                logf,
 		disco:               key.NewDisco(),
@@ -526,9 +526,9 @@ func trySetUDPSocketOptions(pconn nettype.PacketConn, logf logger.Logf) {
 // [magicsock.RebindingConn], which would also remove the need for
 // [singlePacketConn], as [magicsock.RebindingConn] also handles fallback to
 // single packet syscall operations.
-func (s *Server) listenOn(port int) error {
+func (s *Server) listenOn(port uint16) error {
 	for _, network := range []string{"udp4", "udp6"} {
-		uc, err := net.ListenUDP(network, &net.UDPAddr{Port: port})
+		uc, err := net.ListenUDP(network, &net.UDPAddr{Port: int(port)})
 		if err != nil {
 			if network == "udp4" {
 				return err

net/udprelay/status/status.go

@@ -14,8 +14,9 @@ import (
 type ServerStatus struct {
 	// UDPPort is the UDP port number that the peer relay server forwards over,
 	// as configured by the user with 'tailscale set --relay-server-port=<PORT>'.
-	// If the port has not been configured, UDPPort will be nil.
-	UDPPort *int
+	// If the port has not been configured, UDPPort will be nil. A non-nil zero
+	// value signifies the user has opted for a random unused port.
+	UDPPort *uint16
 	// Sessions is a slice of detailed status information about each peer
 	// relay session that this node's peer relay server is involved with. It
 	// may be empty.